β-lactams, such as penicillins and cephalosporins, are the most widely prescribed class of antibiotics. In response to their extensive use and misuse, bacterial resistance has developed. Continued usefulness is threatened by the expression of β-lactamase enzymes, the most widespread resistance mechanism to this class of antibiotics. These enzymes hydrolyze the lactam ring and render the antibiotic inactive against their original cellular targets, bacteria cell wall transpeptidases (FIG. 1).
In an effort to combat these enzymes, β-lactamase inhibitors, like clavulanic acid, and β-lactamase resistant compounds, like the third-generation cephalosporins, have been introduced (FIG. 2). Because these compounds are themselves β-lactams, bacteria responded rapidly to them; existing resistance mechanisms recognize the lactam ring functionality common to both substrates and inhibitors alike. These resistance mechanisms are easily disseminated among bacteria, allowing inhibitor resistance to spread rapidly. Novel inhibitors are required to avoid such pre-evolved resistance mechanisms.
It has been an on-going concern in the art to develop inhibitors that do not chemically and structurally resemble β-lactams, would not be hydrolyzed by β-lactamases, and would not be recognized by sensor proteins that bind β-lactams and upregulate the expression of β-lactamases. Additionally, a useful inhibitor would not be affected by porin channel mutants, which prevent access of β-lactams to their cellular targets. An inhibitor compound chemically and/or structurally dissimilar to a β-lactam minimizes the ability of bacteria to recruit and employ existing resistance mechanisms.
Several classes of non-β-lactam inhibitors of β-lactamases have been identified. Transition-state analog inhibitors, such as boronic acids and phosphonates, inhibit both class A and class C β-lactamases. Intensive study has been devoted to improving the binding affinities of these molecules. One concern with both types of molecules is the covalent adducts formed with activated serine nucleophiles, potentially reducing their specificity.